during which the storage modulus G
0 only slightly decreases. This indicates the
dynamic nature of the cross-link regions in C12M-containing hydrogels as compared
to the chemically cross-linked ones because of the reversible dissociation and
reassociation of the dodecyl side chains. Figure 9b shows frequency dependences
of G
0 (filled symbols) and G
00 (open symbols) of hybrid cross-linked PAAm
hydrogels containing 0–10% n-hexylacrylamide and 1.25 mol% BAAm [95]. At
low frequencies, i.e., at long experimental time scales, they all exhibit similar
mechanical spectra, i.e., G
0 is frequency independent and G
00 is more than an order
of magnitude smaller than G
0 , as typical for strong gels. However, at the high
frequency range, G
00 significantly increases both with the frequency and the
hydrophobe content reflecting energy dissipation due to the reversible nature of
hydrophobic associations that are captured at short experimental time scales. The
length x of the alkyl side chain of n-alkylacrylamides has a similar effect on the
properties of PAAm hydrogels (Fig. 9c) [95]. Increasing the side chain length x at a
fixed hydrophobe content shortens the width of G
0 plateau, i.e., G
00 and G
0 start to
increase at lower frequencies. This reveals increasing lifetime of hydrophobic
associations with increasing alkyl side chain length of the hydrophobic monomers.
Thus, long-lived hydrophobic associations and hence mechanically strong hydrogels
could be generated using long alkyl side chains. However, (meth)acrylates larger
than 12 carbon atoms at their side chains such as n-octadecyl acrylate (C18A) or
docosyl acrylate (C22A) could not be solubilized in monomeric, spherical surfactant
micelles due to their larger sizes as compared to the micelles, hindering their micellar
copolymerization with hydrophilic monomers.
Worm-like micelles (WLMs) formed by self-assembly of surfactant micelles
exhibit interesting rheological properties and a significant solubilization power for
hydrophobes, and thus, they are able to form nano-sized oil depots dispersed in water
[14, 38, 91, 101–103]. A simple way to produce WLMs is the addition of salts such
as NaCl to the aqueous solutions of ionic surfactants which weakens the electrostatic
repulsion between the monomeric micelles and hence promotes their growth to form
“polymer-like” micelles. As shown in Fig. 10a, addition of 1 M NaCl in an aqueous
solution of 7.6 w/v % SDS increases the zero-shear viscosity η o by more than two
orders of magnitude, and simultaneously, the hydrodynamic correlation length ξ H
increases from below 1 to 20 nm due to the micellar growth [102]. Formation of
WLMs provides solubilization of large hydrophobes in SDS-NaCl solutions. For
instance, C18A monomer, which is insoluble in aqueous SDS solutions, could be
solubilized up to 15 w/v % in WLMs formed by the addition of 1.5 M NaCl into
22 w/v % aqueous SDS at 55
C (Fig. 10b) [91]. Interestingly, after solubilization of
the hydrophobes in WLMs, both the zero-shear viscosity and hydrodynamic correlation length ξ H reduce to a low level (Fig. 10c). SANS and cryo-EM measurements
revealed that WLMs undergo a conformational transition from cylindrical to spherical shape after addition of hydrophobes, which is responsible for the decrease in the
zero-shear viscosities [14, 102]. The accumulation of the hydrophobic monomers
inside the core of the micelles creating a curvature on the micelle surface seems to be
responsible for the conformational change in SDS micelles. Moreover, micellar
copolymerization of hydrophilic monomers with a large amount of hydrophobic
38
O. Okay
0 only slightly decreases. This indicates the
dynamic nature of the cross-link regions in C12M-containing hydrogels as compared
to the chemically cross-linked ones because of the reversible dissociation and
reassociation of the dodecyl side chains. Figure 9b shows frequency dependences
of G
0 (filled symbols) and G
00 (open symbols) of hybrid cross-linked PAAm
hydrogels containing 0–10% n-hexylacrylamide and 1.25 mol% BAAm [95]. At
low frequencies, i.e., at long experimental time scales, they all exhibit similar
mechanical spectra, i.e., G
0 is frequency independent and G
00 is more than an order
of magnitude smaller than G
0 , as typical for strong gels. However, at the high
frequency range, G
00 significantly increases both with the frequency and the
hydrophobe content reflecting energy dissipation due to the reversible nature of
hydrophobic associations that are captured at short experimental time scales. The
length x of the alkyl side chain of n-alkylacrylamides has a similar effect on the
properties of PAAm hydrogels (Fig. 9c) [95]. Increasing the side chain length x at a
fixed hydrophobe content shortens the width of G
0 plateau, i.e., G
00 and G
0 start to
increase at lower frequencies. This reveals increasing lifetime of hydrophobic
associations with increasing alkyl side chain length of the hydrophobic monomers.
Thus, long-lived hydrophobic associations and hence mechanically strong hydrogels
could be generated using long alkyl side chains. However, (meth)acrylates larger
than 12 carbon atoms at their side chains such as n-octadecyl acrylate (C18A) or
docosyl acrylate (C22A) could not be solubilized in monomeric, spherical surfactant
micelles due to their larger sizes as compared to the micelles, hindering their micellar
copolymerization with hydrophilic monomers.
Worm-like micelles (WLMs) formed by self-assembly of surfactant micelles
exhibit interesting rheological properties and a significant solubilization power for
hydrophobes, and thus, they are able to form nano-sized oil depots dispersed in water
[14, 38, 91, 101–103]. A simple way to produce WLMs is the addition of salts such
as NaCl to the aqueous solutions of ionic surfactants which weakens the electrostatic
repulsion between the monomeric micelles and hence promotes their growth to form
“polymer-like” micelles. As shown in Fig. 10a, addition of 1 M NaCl in an aqueous
solution of 7.6 w/v % SDS increases the zero-shear viscosity η o by more than two
orders of magnitude, and simultaneously, the hydrodynamic correlation length ξ H
increases from below 1 to 20 nm due to the micellar growth [102]. Formation of
WLMs provides solubilization of large hydrophobes in SDS-NaCl solutions. For
instance, C18A monomer, which is insoluble in aqueous SDS solutions, could be
solubilized up to 15 w/v % in WLMs formed by the addition of 1.5 M NaCl into
22 w/v % aqueous SDS at 55
C (Fig. 10b) [91]. Interestingly, after solubilization of
the hydrophobes in WLMs, both the zero-shear viscosity and hydrodynamic correlation length ξ H reduce to a low level (Fig. 10c). SANS and cryo-EM measurements
revealed that WLMs undergo a conformational transition from cylindrical to spherical shape after addition of hydrophobes, which is responsible for the decrease in the
zero-shear viscosities [14, 102]. The accumulation of the hydrophobic monomers
inside the core of the micelles creating a curvature on the micelle surface seems to be
responsible for the conformational change in SDS micelles. Moreover, micellar
copolymerization of hydrophilic monomers with a large amount of hydrophobic
38
O. Okay